THIS invention relates to a resonant power converter.
A square wave current signal is a preferred type of transmitted signal for making broadband transient decay measurements which, for instance, are used in airborne electromagnetic prospecting systems. A conventional voltage source inverter having transistors and anti-parallel diodes produces an essentially triangular or exponential current waveforms, and is not the best suited for the application. The current source inverter is better used for this type of application, and has been described in E E Ward, "Inverter suitable for operation over a range of frequency", Proc. IEE, Vol. 111, August 1964. A current source inverter typically requires switches with reverse blocking capability and one or more capacitors in parallel to the load. Forced commutation is generally also a feature of such a circuit, as a result of which thyristors are also required.
In Canadian patent 1064584, a pulse generator is disclosed for airborne electromagnetic prospecting. A coil is energised with periodic bipolar current pulses of predetermined amplitude, period and repetition rate and of generally square waveform. A capacitor is connected in parallel to the coil to form a closed oscillatory circuit of predetermined frequency. The oscillatory circuit is controlled via first and second pairs of controlled rectifiers or alternatively connecting and disconnecting the oscillatory circuit from the direct current source and for alternately reversing the direction of flow of current from the direct current source to the coil during successive current pulses The square wave generated by the pulse generator is relatively inflexible, in that it is only capable of making use of the fixed amplitude and frequency components at a given pulse repetition rate making up the square waveform.
One type of voltage source inverter circuit arrangement where a low value capacitor is placed at the input of a transistor inverter is described by J He, N Mohan and B Wold in "Zero-voltage-switching PWM inverter for high-frequency AC-DC power conversion", IEEE Transactions on Industry Applications, Vol. 29, No. 5, September/October 1993, pp 959-968. In this circuit, it is not possible for the resonant capacitor voltage to become larger than the source voltage. In addition, resonance does not take place between the load and the resonant capacitor, but rather between the capacitor and an auxiliary inductor.
In a further circuit by A Hava, V Blasto and T A Lipo, described in "A modified C-dump converter for variable reluctance machines", 1991 IEEE IAS Conference Record, pp 886-891, unipolar pulses are provided for the windings of a reluctance motor. A smaller capacitor is provided which does not resonate with the load, and the diode in series with the voltage source is not connected to the DC side of the inverter, but rather directly to the load windings.
In applications such as airborne electromagnetic prospecting, it is desirable for there to be relatively flexible control of the electromagnetic signal emanating from the coil or loop.
In U.S. Pat. No. 4,410,926 a circuit is disclosed for generating DC magnetic fields of alternating polarity. A low value capacitor which can resonate with the load is placed at the input of an inverter. However, the wave shape control is very limited and does not give the flexibility which is required for new generation geophysical detection systems. The polarity changes are invariably implemented using relatively slow half wave resonant transitions and the current amplitude is not actively controlled.